a) Turn on debug logging to store errors to a log file
--older than 1.3.9--
Install the Debug Error-Logging Utility (note: this is ALREADY installed in v1.3.9x)
Click {here} to download it. (note: it's already installed in v1.3.9x)
If you're dealing with blank admin pages, for the upload destination, substitute the /admin/includes/extra_configures folder instead of /includes/extra_configures where mentioned in the instructions.
--older than 1.3.9--
The debug tool will create files in your /cache/ folder, named "myDebug-xxxxxx.log" (or "myDebug-adm-xxxxxxx.log" if they are from the admin side).
You'll want to read the contents of these files to see what the actual PHP errors are.
Then search this FAQ area for your error message and/or see the "Dealing with Error Messages" section, below ...